1. Field of the Invention
The present invention relates generally to a user allocating apparatus and method in a multiple antenna mobile communication system, and in particular, to an apparatus and method for allocating a user in a multiple antenna mobile communication system supporting multi-user diversity.
2. Description of the Related Art
Unlike a wired channel environment, a radio channel environment typically has low reliability due to multipath interference, shadowing, wave attenuation, time-variant noise, and interference. The low reliability is an obstacle to increasing data rate. Major examples of many techniques developed to overcome this problem are error control coding and diversity. The former suppresses the effects of signal distortion and noise and the latter combats fading by independently receiving a plurality of signals experiencing fading.
Diversity schemes are categorized into time diversity, frequency diversity, multipath diversity, and spatial diversity. Time diversity achieves diversity in time by combining channel encoding with interleaving. In frequency diversity, signals are transmitted at different frequencies and thus in different paths. Multipath diversity is a scheme of achieving diversity by distinguishing multipath signals using different fading information. Spatial diversity is characterized in that both a transmitter and a receiver use a plurality of antennas and achieve diversity by using independent fading signals. Spatial diversity uses an antenna array.
However, the error control coding technology and the diversity schemes for radio channels are not effective in satisfying the demands for high-speed data service, such as Internet connection and multimedia service. Since provisioning of the high-speed data service requires increased frequency efficiency, a mobile communication system having an antenna array is studied correspondingly.
An antenna array system seeks to increase frequency efficiency in the space domain using multiple antennas at the transmitter and the receiver. Considering that time and frequency resources are limited, it is easy to further increase data rate by utilizing the space domain. Such antenna array systems are Vertical Bell Lab Layered Space Tome (V-BLAST) proposed by Bell Labs and space division multiplexing. Since each antenna transmits information independently, these antenna array systems are essentially Multi-Input Multi-Output (MIMO) systems.
To increase capacity with high frequency efficiency, the channels between transmit antennas and receive antennas must have small correlation coefficients in the antenna array systems, so that information transmitted from the transmit antennas experience different channels and thus can be identified at a Mobile Station (MS). That is, a signal transmitted from each transmit antenna must have a different spatial characteristic, for identification at the MS, thereby leading to an increase of channel capacity. The antenna array systems are suitable for multipath signals with different spatial characteristics. However, the increase of capacity is not great in a Line Of Sight (LOS) environment, as compared to a single transmit/receive antenna system. Hence, the antenna array systems are feasible in a rich-scattering environment with multipath signals and low correlation coefficients between the transmit and receive antennas, i.e. in a diversity-effective environment.
The use of an antenna array at the transmitter/receiver generally increases channel capacity. The channel capacity is dependent on whether the transmitter/receiver has knowledge of channel information. If both the transmitter and the receiver have channel information, the channel capacity is highest, and if neither the transmitter nor the receiver has channel information, the channel capacity is lowest. If only the receiver has channel information, the channel capacity increase is between the two cases.
To acquire the channel information, the transmitter estimates channel condition on its own, or receives feedback information about the channel condition. The channel information required in the antenna array system is the channel response between the transmit antennas and the receive antennas. Therefore, the channel information increases in proportion to the number of the transmit/receive antennas.
In the antenna array system, including multiple transmit/receive antennas, the channel capacity increases in proportion to the number of available antennas in the transmitter/receiver. On the other hand, in the case of feedback of the channel information, the amount of the feedback information increases with the number of the antennas. Accordingly, a need exists for increasing the channel capacity with a reduced amount of feedback information.
As previously described, a conventional Spatial Multiplexing (SM)-MIMO system adopts V-BLAST for reception. V-BLAST relies on interference cancellation which is performed by forward ordering detection or reverse ordering detection. In the forward ordering detection, a subchannel with the highest Signal-to-Interference Noise Ratio (SINR) is selected first of all at each step. The forward ordering detection is suitable for Equal Power (EP) and Equal Rate (ER) for each subchannel. In case of optimal bit allocation and power allocation, the reverse ordering detection first detects a channel with a lower SINR. That is, detection is performed in an ascending order of SINR.
FIG. 1 is a block diagram of a conventional SM-MIMO system using forward ordering detection and reverse ordering detection. The SM-MIMO system using adaptive modulation detects a signal on a substream-by-substream basis by V-BLAST. An effective Signal-to-Interference Ratio (SIR) and bit loading calculator selects the forward ordering or the reverse ordering according to SIR. The forward ordering and the reverse ordering are expressed as formulas in FIG. 1, which are computed to determine an optimal detection order in the effective SIR and bit loading calculator. The forward ordering is based on the assumption of EP and ER, whereas the reverse ordering is based on the assumption of Power Allocation (PA) and Adaptive Rate (AR). The effective SIR and bit loading calculator provides the determined optimal detection order to a V-BLAST portion. The V-BLAST portion receives signals through a plurality of antennas according to the optimal detection order.
These conventional forward and reverse ordering detection techniques take into account only SINR in Successive Interference Cancellation (SIC). As a result, they are not efficient in terms of minimization of an average error probability determined by the minimum symbol distance of a modulation and a received SINR. Moreover, they operate for a single user and thus not suitable for a mobile communication system with multi-user diversity. Therefore, scheduling for a plurality of users is required to optimize error probability performance to support multi-use diversity in the conventional multiple antenna system.